Cereal grains contain three groups of important biopolymers: starch, proteins and non-starch polysaccharides. Starch and a large part of the protein fraction are located in the endosperm and serve as reserve material for the plant during germination and the initial stages of growth. They are degraded by amylases and proteases respectively [1]. The non-starch polysaccharides include mainly arabinoxylan (AX) and β-glucan which are part of the cell walls and are hydrolysed by xylanolytic and β-glucanolytic enzymes respectively [1, 2]. The degradation of these cell wall polysaccharides in the endosperm and aleurone layer during the germination improves the accessibility of starch and protein for amylases and proteases [3, 4]. Proteins that inhibit amylases [5-9] and proteases [10-13] have already been purified from cereals and have been characterised extensively. They possibly regulate the plant starch and nitrogen metabolism and/or play an important role in plant defence by inhibiting enzymic hydrolysis by micro-organisms and predators.
Recently, a new class of enzyme inhibitors, i.e. proteinaceous inhibitors of endo-β-1,4-xylanases (endoxylanases, EC: 3.2.1.8), has been discovered in cereals by Debyser and Delcour [14] and Debyser et al. [15]. Inhibition activity against such xylanolytic enzymes was found in different cereals such as wheat (Triticum aestivum L.), barley (Hordeum vulgare L.) and rye (Secale cereale L.) [14, 16].
An endoxylanase inhibitor, named TAXI (T. aestivum L. endoxylanase inhibitor), was purified from wheat flour and characterised by Debyser and Delcour [14] and Debyser et al. [17] wherein for the screening of the inhibition activity a single endoxylanase from Aspergillus niger was used. TAXI has a molecular mass of ca. 40.0 kDa and occurs in two molecular forms A and B, B presumably as a result of proteolytic modification of A [14, 16, 17]. As a result of reduction with β-mercaptoethanol, the modified molecular form B dissociates in two fragments with molecular masses of ca. 10.0 and 30.0 kDa respectively, whereas the molecular mass of the non-modified form does not change upon reduction. The inhibitor is heat sensitive and has a pI of ca. 8.8 [14, 16, 17]. Rouau and Surget [18] also found evidence for the presence of endoxylanase inhibitors in regular and durum wheats. These authors detected high inhibition activity against microbial endoxylanases in both wheat flour and bran. McLauchlan et al. [19] and Hessing and Happe [20] purified a wheat endoxylanase inhibitor structurally quite different from TAXI. In the isolation procedure [19], an endoxylanase, partially purified from a commercial A. niger hemicellulase preparation was used for the screening of the inhibition activity. The resulting inhibitor is monomeric, glycosylated and a heat sensitive protein. It has a pI of 8.7-8.9 [19] or higher than 9 [20], a molecular mass of 29.0 kDa [19] and 31.0 kDa [20] and was found to be a competitive inhibitor. The N-terminal amino acid sequence is 87% identical with a sequence close to the N-terminus of the rice chitinase III polypeptide chain and shows no homology with the amino acid sequences of TAXI [19,20].Further information on TAXI can be found in Sibbesen and Sørensen [36].